Federal Acquisition Regulations (FARs) are rules governing purchases made by the executive agencies of the U.S. government. FARs are discussed extensively in Government Contracts Under The Federal Acquisition Regulation (West). For questions, you can call the FAR Staff at the GSA, write to firstname.lastname@example.org and/or see out the FAQs on the GSA's Ask Acquisition Questions & Answers page.
FARs are officially published in Chapter 1 of Title 48 of the Code of Federal Regulations (CFR). Unofficially, FARs are published in looseleaf form in the GSA's Federal Acquisition Regulation and CCH's Government Contracts Reporter, and in the Federal Acquisition Regulation Desk Reference. In addition, FARs are posted free in the Federal Acquisition Regulation section of Acquisition Central. FARs are searchable on Lexis (PUBCON;FAR) and Westlaw (FAR).
When a FAR is amended or a new FAR is adopted, the GSA puts out a Federal Acquisition Circular (FAC), which is published in the Federal Register and prepared as replacement pages for the GSA looseleaf. Circulars back to 1995 are posted on Acquisition Central's Federal Acquisition Regulation page in both formats. You can get older FARs from back issues of the Federal Register or the "FAR Archives" in CCH's subscription-based Government Contracts Library.
Changes are also promptly incorporated into the the eCFR and CCH's Government Contracts Reporter. By contrast, the official CFR (print or online) is updated only annually, so official CFR users should also check recent Circulars or the List of CFR Sections Affected.
Proposed FARs: Proposed FARs are posted on Acquisition Central's Interrim and Proposed Rules page.
Defense FARS: Department of Defense Acquisition Regulations, known as the Defense Acquisition Regulation Supplement (DFARS) are published in Title 48 of the Code of Federal Regulations, Chapter 2, and are included in all current and historical print and electronic editions of the CFR (see the "Code of Federal Regulations" entry). They are also posted free at FARSite. For advanced features, use Lexis (PUBCON;D0D), Westlaw (DRARS) or, if you have access, the CCH Government Contracts Reporter.
Note: Additional Defense Acquisition Regulations are published in CFR Title 32, Subtitle A, Chapter I, Subchapter A (Acquisition).
Procedures, Guidance and Information (PGI): The PGI supplements the Defense FARs. "Relevant procedures, guidance, and information that do not meet the criteria for inclusion in the DFARS are issued in the DFARS companion resource, PGI. Unclassified, non-confidential memoranda, guidance, and other DPAP procurement-related policy documents can be found in the appropriate PGI subpart" (from About DFARS and PGI). You can get PGIs through this chart. They are also available in the subscription-based CCH Government Contracts Reporter (go to "Regulations - Agency FAR Supplements - Department of Defense FAR Supplement").
Other Acquisition Regulations: Acquisition regulations by/for individual agencies are published in Chapters 2 through 99 of CFR Title 48.